Drilling fluids are employed in the drilling of oil and gas wells, or other wells. These fluids are circulated down the drill pipe and up the annulus between pipe and formation and serve several functions, the principal ones being removing cuttings, cooling and lubricating the bit, and stabilizing the formation. The most commonly used fluids are "drilling muds" wherein colloidal clays, barites, and similar inorganic solids, at levels of 25-35 pounds per barrel (ppb), a barrel being 42 U.S. gallons, provide thickening and borehole stabilization. These muds have several disadvantages, the more serious ones being appreciably slower drilling rates compared to water alone, inability to stabilize troublesome formations such as heaving and swelling shales, and forming of a filter cake of clay upon loss of water into the formation, (thus clogging the borehole) which interferes with drilling and movement of the drill pipe. For many years, polymers have been added to inorganic muds to modify the properties of the clay such as reducing water loss or increasing thickening; however, the basic limitations of clay based drilling muds remain. More recently, low solids drilling fluids based principally on soluble polymers, with little or no colloidal clays, have been introduced or proposed. Low molecular weight polymers such as of acrylic acid, methacrylic acid and the like, and salts thereof, have been used to modify clay muds; these low molecular weight products (below about 75,000 Mv) do not function as thickeners, and may even detract from the desired properties in a drilling fluid. These soluble polymers, however, have not found universal acceptance because the polymers have not provided all the properties required in a complete drilling fluid. Specifically, it has been found that they do not give optimum effective thickening of fresh and brine waters. Fluids have also been proposed which are essentially free of clay and in which the necessary viscosity is achieved by means of macromolecular polysaccharides, or derivatives thereof. It has been found that the polysaccharides do not stabilize swelling shales and the like, to a great extent.
Examples of references showing the use of either soluble polymers or soluble polysaccharide resins or gums in drilling fluids appear in U.S. Pat. Nos. 3,699,042, 3,852,201, and 3,654,164. Pen'kov et al, Kollordnyii Zhurnal, 35, No. 4, pages 799-801, July, 1973, describe the use of copolymers in drilling fluids, and in other drilling fluids disclose polysaccharides, but not the combination of the two. C.A. 75:119791x shows graft copolymers of carboxymethyl cellulose or alginates with "polyacrylates", apparently in clay-containing drilling muds, as stabilizers. C.A. 60:11808e shows modifying gypsum muds with an acrylate-acrylamide copolymer or with a carboxymethyl cellulose, and discusses the effect of high temperatures on each of these thickeners. In addition to these disclosures, U.S. Pat. No. 3,284,353 discloses carboxymethyl cellulose in drilling muds, Pat. No. 3,654,164 discloses a copolymer of a vinyl ether and maleic anhydride, hydroxyethyl cellulose is disclosed in U.S. Pat. No. 3,852,201, a styrene-maleic anhydride copolymer is disclosed in U.S. Pat. No. 3,332,872, and a copolymer of acrylic acid and acrylamide is disclosed in U.S. Pat. No. 3,323,603. None of these, however, suggest the advantages discovered in accordance with the present invention of using the specified combination of an acid-containing polymer and of a polysaccharide such as hydroxyethyl cellulose, particularly having the specific materials, proportions, and properties of the materials of the invention.